1. Field of the Invention
The present invention relates to a method of manufacturing a carrier, a carrier, a development agent, and a process cartridge.
2. Description of the Background Art
In electrophotography, images are formed by forming a latent electrostatic image by electrostatic charge on an image bearing member formed of photoconductive materials, etc., attaching charged toner particles to the latent electrostatic image to obtain a toner image, transferring the toner image to a recording medium such as paper, and fixing the toner image.
In recent years, the technologies regarding photocopiers and printers employing electrophotography have been rapidly diffusing from monochrome field to full color field, thereby expanding the full color market.
In color image forming employing full color electrophotography, four color toners containing toners of three primary colors of yellow, magenta, and cyan and black are laminated for representing all the colors.
Therefore, to obtain vivid full color images with excellent color representation, the surface of the fixed toner image is smoothed in some degree to reduce light scattering. Therefore, most of the image gloss of images produced by a typical full color photocopier, etc. ranges from middle gloss to high gloss, e.g., 10 to 50%.
In general, as a method of fixing dry toner images on a recording medium, a contact heating fixing method is commonly used in which a heated roller or belt is pressed against the toner.
This method is advantageous in terms of heat efficiency, speed of fixing, imparting gloss and transparency to color toner. However, in this method, since melted toner is peeled off from the surface of a heated fixing member after the melted toner contacts the surface under pressure, part of the toner image may be attached to the surface, resulting in transfer of the attached toner to another recording medium on which another toner image is formed. This is referred to as offset phenomenon.
To prevent the offset phenomenon, in general, a heating fixing member having a surface formed of materials having excellent releasing property such as silicone rubber and fluorine resins is used and in addition releasing oil such as silicone oil is applied to the surface of the heating fixing member.
However, this method is extremely effective to prevent offset of toner but requires a device to supply the releasing oil, thereby not shrinking but expanding the size of a fixing device. Therefore, a monochrome toner tends to be used in a method in which the viscous elasticity of the toner in melted state is improved by adjusting the molecular weight distribution of the binder resin to prevent fracturing of the melted toner from inside and furthermore a releasing agent such as wax is contained to apply no (oil free) or few amount of releasing oil to a fixing roller.
Similarly, such an oil-free application has become common for color toner in terms of size reduction of a machine and simplification of the structure thereof.
However, as described above, since the surface of a fixed image is required to be smooth to improve the color representation of color toner, toner in melted state having a low viscous elasticity is suitable. Therefore, color toner causes offset phenomenon more easily than monochrome toner, which does not require gloss. That means that it is more difficult to employ a method of applying no (oil free) or few amount of releasing oil to a fixing roller in the case of color toner. In addition, when a toner containing a releasing agent is used, the attachability of the toner increases, thereby degrading the transfer property of the toner to a transfer medium and resulting in contamination of a triboelectric member such as carriers by the releasing agent, which leads to degradation of the chargeability and durability. Furthermore, in consideration of the environment, toner having a low fixing temperature has been dominantly used to reduce the power consumption. Such toner tends to be inferior to typical toner in terms of preservability in a high temperature and high moisture environment.
In addition, carriers have been used to meet the demand for forming more beautiful images faster. As the speed of image forming increases, a development agent containing carrier and toner particles are subjected to increasing stress, which leads to a short working life of the carrier although carriers have a long working life in general.
Furthermore, to produce higher quality images, it is effective to reduce color contamination ascribable to the color deriving from the carrier covering resin. However, if the covering resin is heated and processed in a low temperature to reduce the color contamination, the amount of residual solvent in the covering resin tends to increase and the degree of the cross-linking in the covering resin tends to decrease, which leads to solidification (i.e., a phenomenon in which carrier and toner particles stick together to form blocks) in a high temperature and high moisture environment.
For example, when a fresh development agent is placed in a development unit, the toner easily falls from a case that contains the toner to the development unit but the solidified development agent does not easily drop so that the development agent is not transferred into the unit sufficiently and therefore defective images may be produced. In addition, if the solidified blocks remain in the development agent that has been transferred into the unit, the blocks are caught at a regulation board where the amount of the development agent on the development roller is uniformly adjusted.
Thus, the development agent is hauled onto some part of the surface of the development roller but not the other part thereof, resulting in production of defective images. Moreover, if the development agent is solidified in the development unit or the development agent is placed in the development unit in the first place and solidified to a degree beyond which the stirring wing in the development unit does not crumble the solidified blocks, problems arise such that defective images are easily produced and the development unit is unable to function.
One of the deduced mechanism of such problems is that the specific surface of the carrier and the toner increases as the particle diameter of the carrier and the toner decreases. Also such problems can be ascribable to low temperature fixing and oil-free application. Furthermore, other inferable factors causing problems deriving from solidification are consequential size reduction of a development unit to meet the demand for size reduction of an entire machine and difficulty of designing of falling of a development agent.
Moreover, as a result of diversification of the market, the use of a product is required to be guaranteed in a wide range of environments. Consequently, a development agent having a tough quality is demanded.
To solve these problems, Japanese patent application publication No. (hereinafter referred to as JP-A) 2006-154453 describes a method of raising the baking temperature of resin covered carriers in an electric furnace to improve the problem of solidification. However, the covered resin baked at a higher temperature tends to have blackish color. Therefore, this method is not suitable for the color contamination problem.
The solidification problem that has not happened before is now recognized as an issue for a development agent. By contrast, solidification of toner has been already recognized. For example, JP H02-051177-A, JP H09-304959, JP-H09-329910-A, and JP 2001-312093 describe the solidification of simple toner but not a development agent containing toner and carrier.
To deal with such problems, for example, JP 2007-102159 and JP 2008-70837 describe carriers that contain particles larger than the film covering the carriers. Furthermore, JP 2007-286078-A describes a method of performing development while discharging extra development agent in the development unit described above in addition to replenishment of toner and carrier to the development unit.
In addition, with regard to carriers, the demand for forming more beautiful images faster is strong as described above. Therefore, the development roller is driven to rotate faster and faster in line with appearance of a high speed machine of late, resulting in an increase of centrifugal applied to the development agent.
The particle diameter of carrier and toner has been markedly reduced to meet the demand for improvement on the quality of images.
Therefore, the particle diameter is irrelevant to an increase of the centrifugal caused by high speed performance as long as the magnetization remains unchanged (magnetization per weight is the same). However, with regard to size reduction of toner and carrier particles for improvement on the quality of images, since the surface area per unit weight increases as the particle diameter decreases, the amount of charge increases against magnetization.
That is, the carrier tends to be weakly attracted by the electric force of the image bearing member so that carrier attachment easily occurs.